Larry H. Weaver

A Model Binding Site for Testing Scoring Functions in Molecular Docking

Prediction of interaction energies between ligands and their receptors remains a major challenge for structure-based inhibitor discovery. Much effort has been devoted to developing scoring schemes that can successfully rank the affinities of a diverse set of possible ligands to a binding site for which the structure is known. To test these scoring functions, well-characterized experimental systems can be very useful. Here, mutation-created binding sites in T4 lysozyme were used to investigate how the quality of atomic charges and solvation energies affects molecular docking. Atomic charges and solvation energies were calculated for 172,118 molecules in the Available Chemicals Directory using a semi-empirical quantum mechanical approach by the program AMSOL. The database was first screened against the apolar cavity site created by the mutation Leu99Ala (L99A). Compared to the electronegativity-based charges that are widely used, the new charges and desolvation energies improved ranking of known apolar ligands, and better distinguished them from more polar isosteres that are not observed to bind. To investigate whether the new charges had predictive value, the non-polar residue Met102, which forms part of the binding site, was changed to the polar residue glutamine. The structure of the resulting Leu99Ala and Met102Gln double mutant of T4 lysozyme (L99A/M102Q) was determined and the docking calculation was repeated for the new site. Seven representative polar molecules that preferentially docked to the polar versus the apolar binding site were tested experimentally. All seven bind to the polar cavity (L99A/M102Q) but do not detectably bind to the apolar cavity (L99A). Five ligand-bound structures of L99A/M102Q were determined by X-ray crystallography. Docking predictions corresponded to the crystallographic results to within 0.4A RMSD. Improved treatment of partial atomic charges and desolvation energies in database docking appears feasible and leads to better distinction of true ligands. Simple model binding sites, such as L99A and its more polar variants, may find broad use in the development and testing of docking algorithms.

The refined structures of goose lysozyme and its complex with a bound trisaccharide show that the "goose-type" lysozymes lack a catalytic aspartate residue.

The structure of goose egg-white lysozyme (GEWL) has been refined to an R-value of 15.9% at 1.6 A resolution. Details of the structure determination, the refinement and the structure itself are presented. The structure of a complex of the enzyme with the trisaccharide of N-acetyl glucosamine has also been determined and refined at 1.6 A resolution. The trisaccharide occupies sites analogous to the B, C and D subsites of chicken (HEWL) and phage T4 (T4L) lysozymes. All three lysozymes (GEWL, HEWL and T4L) display the same characteristic set of bridging hydrogen bonds between backbone atoms of the protein and the 2-acetamido group of the saccharide in subsite C. Glu73 of GEWL is seen to correspond closely to Glu35 of HEWL (and to Glu11 of T4L) and supports the established view that this group is critically involved in the catalytic mechanism. There is, however, no obvious residue in goose lysozyme that is a counterpart of Asp52 of chicken lysozyme (or of Asp20 in T4L), suggesting that a second acidic residue is not essential for the catalytic activity of goose lysozyme, and may not be required for the activity of other lysozymes.

Corepressor-induced organization and assembly of the biotin repressor: a model for allosteric activation of a transcriptional regulator.

The Escherichia coli biotin repressor binds to the biotin operator to repress transcription of the biotin biosynthetic operon. In this work, a structure determined by x-ray crystallography of a complex of the repressor bound to biotin, which also functions as an activator of DNA binding by the biotin repressor (BirA), is described. In contrast to the monomeric aporepressor, the complex is dimeric with an interface composed in part of an extended β-sheet. Model building, coupled with biochemical data, suggests that this is the dimeric form of BirA that binds DNA. Segments of three surface loops that are disordered in the aporepressor structure are located in the interface region of the dimer and exhibit greater order than was observed in the aporepressor structure. The results suggest that the corepressor of BirA causes a disorder-to-order transition that is a prerequisite to repressor dimerization and DNA binding.

Competing protein:protein interactions are proposed to control the biological switch of the E coli biotin repressor

A model is suggested for the complex between the biotin repressor of Escherichia coli, BirA, and BCCP, the biotin carboxyl carrier protein to which BirA transfers biotin. The model is consistent with prior physical and biochemical studies. Measurement of transfer rates for variants of BirA with single‐site mutations in the proposed BirA:BCCP interface region also provides support. The unique feature of the proposed interaction between BirA and BCCP is that it uses the same β‐sheet region on the surface of BirA that the protein uses for homodimerization into a form competent to bind DNA. The resulting mutually exclusive protein:protein interfaces explain the novel feature of the BirA regulatory system, namely, that transcription of the genes involved in biotin synthesis is not determined by the level of biotin, per se, but by the level of unmodified BCCP. The model also provides a role for the C‐terminal domain of BirA that is structurally similar to an SH3 domain.

Multiple methionine substitutions are tolerated in T4 lysozyme and have coupled effects on folding and stability

In order to further explore the tolerance of proteins to amino acid substitutions within the interior, a series of core residues was replaced by methionine within the C-terminal domain of T4 lysozyme. By replacing leucine, isoleucine, valine and phenylalanine residues a total of 10 methionines could be introduced, which corresponds to a third of the residues that are buried in this domain. As more methionines are incorporated the protein gradually loses stability. This is attributed in part to a reduction in hydrophobic stabilization, in part to the increased entropic cost of localizing the long, flexible methionine sidechains, and in part to steric clashes. The changes in structure of the mutants relative to the wildtype protein are modest but tend to increase in an additive fashion as more methionines are included. In the most extreme case, namely the 10-methionine mutant, much of the C-terminal domain remains quite similar to wildtype (root-mean-square backbone shifts of 0.56 A), while the F and G helices undergo rotations of approximately 20 degrees and center-of-mass shifts of approximately 1.4 A. For up to six methionine substitutions the changes in stability are additive. Beyond this point, however, the multiple mutants are somewhat more stable than suggested from the sum of their constituents, especially for those including the replacement Val111-->Met. This is interpreted in terms of the larger structural changes associated with this substitution. The substituted sidechains in the mutant structures have somewhat higher crystallographic thermal factors than their counterparts in WT*. Nevertheless, the interiors of the mutant proteins retain a well-defined structure with little suggestion of molten-globule characteristics. Lysozymes in which selenomethionine has been incorporated rather than methionine tend to have increased stability. At the same time they also fold faster. This provides further evidence that, at the rate-limiting step in folding, the structure of the C-terminal domain of T4 lysozyme is similar to that of the fully folded protein.